Radio signal rectifying and amplifying system



April 5, 1938. 5. L. BEERS 13,127

RADIO SIGNAL RECTIFYING AND AMPLIFYING SYSTEM Filed July 51, 1935 Gmgge L13 66m,

WITNESS Y .J

MT wE/v' K Patented Apr. 5, 1938 UNITED STATES RADIO SIGNAL RECTIFYING AND AMPLI- FYING SYSTEM George L. Beers, Gollingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 31, 1935, Serial No. 34,005

1 Claim.

The present invention relates to a radio signal rectifying and amplifying system, and more particularly, to a system of that character in which an electric discharge amplifier directly follows 5 in the signal circuit, a diode detector for rectifying the signals.

It is a primary object of the present invention to provide an improved diode rectifier and signal amplifier therefor wherein the amplifier may be coupled directly to the diode rectifier circuit without signal distortion.

It is a further object of the present invention to provide in a radio signalling system, an improved coupling circuit whereby an electric discharge amplifier device may be coupled directly to a diode rectifier without the usual coupling condenser and grid leak combination and without signal distortion.

In a coupling circuit between the grid of a signal amplifier and the output circuit of a diode rectifier device it is preferable to provide biasing potential for the amplifier from the diode by signal rectification, and in doing so a signal path in shunt to the diode output circuit can be avoided.

As is well known, current may begin to fiow in a diode rectifier circuit, through the diode rectifier, beforev the anode becomes positive, that is, while it may have a slight negative potential with respect to the diode cathode. Likewise, an amplifier control grid may also draw current while it is still negative with respect to cathode.

It has been found that with the diode connected to provide biasing potential to an amplifier grid as well as rectified signals, distortion may further be minimized with this arrangement, when the anode of the diode rectifier is arranged to start to draw current at a more negative potential than the control grid of the triode or following amplifier. This is particularly desirable in connection with signals having high percentage modulation.

There are several radio tubes commercially available which are provided with diode anodes in addition to triode and pentode amplifier elements, having a common cathode in a single envelope, but, since the diode and amplifier elements make use of the same cathode, the tube construction determines whether the control grid of the amplifier, for example, or the anode of the diode will take current at the more negative potential. It has been found that under certain operating conditions, the grid of a double diode triode, for example, may take current at a more negative potential than the anode of the diode,

and this has been found to'result in signal distortion.

Accordingly, it is an object of the present invention to provide an improved coupling system between a diode rectifier and an electric discharge amplifier, wherein the grid may be conductively coupled directly to the diode output circuit, whereby the diode anode may start to take current at a more negative potential than the grid of the amplifier. 10

It is also a further object of the present invention to provide an improved coupling circuit for an electric discharge amplifier device having a control grid, whereby saidcontrol grid may receive an initial biasing potential and a signal 15 controlled biasing potential for meeting signal peaks without distortion, and for limiting the anode current of the amplifier device when no signals are amplified.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claim.

.In the drawing, Fig. 1 is a schematic circuit 25 diagram of an audio frequency amplifier having a diode signal rectifier connected therewith to provide'a detecting and amplifying system embodying the invention; and

Fig. 2 is a similar schematic circuit diagram of 30 a modification of the circuit of Fig. 1 also embodying the invention.

Referring to Fig. 1, 5 is a diode rectifier device of the electric discharge type having a cathode I provided with an equi-potential heater means 9- 35 and an anode electrode II. Signals to be rectified are applied'to the rectifier through a suitable input circuit, such as a tuned circuit comprising the secondary 13 of a signal input transformer l5 and a shunt tuning condenser 16. One 40 terminal ll of the tuned circuit is connected through a rectifier output impedance or resistor 23 to the cathode I through a lead 25. A high frequency by-pass condenser 21 is connected across the resistor 23. The opposite input circuit terminal 21 is connected through a lead l9 with the diode anode l I.

Output leads for the rectified signal component and direct current resulting from rectifica- 50 tion are indicated at 29 and 31 and a variable tap connection 33 for the lead 3| is provided to adjust the signal and direct current output potentials. The lead 29 is connected with the positive terminal indicated at 35. The negative ter- 55 minal for the diode output impedance is indicated at 31.

Signals are applied to the input circuit through the input transformer I 5 which may be connected with leads 43 representing any suitable connection for signal supply. In the present example, the transformer l5 may be considered as the output transformer of an intermediate frequency amplifier (not shown).

The rectifier device 5 or second detector is coupled to an audio frequency amplifier comprising two electric discharge amplifier devices 41 and 49 in the first and second stages thereof respectively, the latter devicebeing coupled to a suitable sound producing means such as a loudspeaker 5! through an output coupling transformer 53. p 7

The first stage amplifier'device is connected with the diode output impedance 23 through-the output leads 29 and 3 I. The arrangementis such that the amplifier device 41 may receive rectified signals and direct current biasing potential from the diode output impedance 23. Furthermore, in accordancewith the invention, the control grid 55 is connected through a grid or input lead 51 with the lead 3| while the cathode 59 is connected through a self-bias resistor 6| with the lead 29. With this arrangement, the diode output impedance 23 having a polarity as indicated, the control grid 55 may receive an increasing negative biasing potential with increase in signal strength. The rectified signal component and direct current component are applied between the grid and the cathode'fro'm a portion of the output resistor located between the movable contact 33 and the terminal 35. The contact 33 may, therefore, be adjusted to vary the signal output from the receiving system.

It will be noted that the self-bias resistor 6| is suitably by-passed by a condenser 63 of low impedance to audio frequency currents. The grid and cathode of the amplifier are therefore substantially directly connected with the diode output impedance 23. By this arrangement the audio frequency impedance of signal input circuit for the grid 55 is the same as the D. C. impedance of the circuit.

The firststage amplifier device 41 is coupled to the output amplifier device 49 through an impedance coupling system comprising a suitable audio frequency choke coil or anode inductance 65 and a coupling condenser 51. i

The impedance 65 may be utilized to provide a step-up transformer by providing a tap 69 intermediate of the terminals H and 13 to which the anode 15 may be connected alternatively as shown by a selector switch 11 through a connection lead I9. When the switch is connected to the terminal H, the inductance'65 is utilized as an anode coupling impedance connected through the coupling condenser 61 with the input circuit for the amplifier 49 and the control grid 8| thereof which input circuit includes a grid leak resistor 83.

When the switch is moved to the contact providing connection with the lead 19 and the tap terminal 69, the inductance 65 becomes an autotransformer providing a step-up ratio and is coupled through thecondenser '61 in the same manner as above described.

,Anodef'potentials are derived from a supply source comprising a voltage divider resistor 85 having direct current supply terminals 81 with polarities, as indicated. The anode circuit of the device 49 is connected through a lead 89 with a positive tap 9|. The cathode 93 is connected through a. lead 95 with a more negative tap point 91. The cathode' 59 is also connected with the tap point 91 through the self-bias resistor 6|. The terminal 13 is connected with a tap point 99 more positive than the tap point 91, thereby to energize the anode 15 at a suitable positive potential. The grid leak resistor 83 is connected through a lead In! with a tap point I03 more negative than the tap point 91, thereby to place a suitable negative fixed biasing potential on the rid 8|.

It will be noted that the first stage amplifier device 41 is a triode amplifier and may be of the low-mu type. Therefore, it is preferable to utilize in the anode circuit thereof as a coupling means with the second stage amplifier device, a

step-uptransformer, and preferably an autotransformer as shown having a suitable step-up ratio,- as provided by the tap connection on the inductance 65.

The operation of the system described, is as follows: The device 5 operates as a diode rectifier or audio frequency detector in connection with the device 41 as a separate amplifier, which receives on its grid 55 a portion or all of the voltage developed across the diode output resistor or impedance 23, directly through the lead 51 since the cathodes of both the rectifier and the amplifier are connected together through the lead 29. The elimination of coupling condensers and grid leak resistors results in making the audio frequency and direct current resistance of the input circuit equal, thereby aiding in the reduction of distortion. The self-bias resistor additionally in the cathode lead of the amplifier device 41 is of a value to provide sufilcient negative bias on the amplifier grid 55 to prevent the grid from drawing current at the peaks of the audio or rectified signals normally supplied to the grid by the diode rlectifier 5, particularly when the modulation is i'gh.

The self-bias resistor 6 I, furthermore, prevents the anode current of the amplifier tube 41 from reaching excessive values when the system is receiving zero signals and the biasing potential derived from the diode circuit output resistor is zero.

Withthe arrangement shown, it will be seen that the anode ll of the diode rectifier 5 must initially draw current in response to signals at a more negative potential than the grid of the triode 4'! draws current, thereby substantially minimizing distortion.

In other words, the grid of the triode or amplifier should not draw current when no signals are impressed on the diode.

. Current will fiow in the grid of the triode, for example, when grid is .1 to .75 volt negative with respect tocathode. In a diode circuit a current may flow when the anode is as much as .5 volt negative with respect to the cathode. Therefore, when the control grid of the amplifier device is directly connected to the output impedance of the diode, it is necessary that an additional negative potential be provided for the grid of the amplifier device to insure that the control grid of the amplifier device does not take current, when the peak audio 7 frequency potential developed across the diode output current or impedance is approximately equal to the direct current potential developed thereacross.

Thesignals applied to the amplifier device 41 are transmitted through the step-up transformer provided by the inductance 65, with the switch 11 in position toconnect it as an auto-transformer,

and are further amplified in the fixed bias amplifier 49 and applied to the output device 5|. The fixed bias permits higher power output and the diode biased amplifier with fixed initial bias and direct coupling permits distortionless amplification preceding the output stage.

Furthermore, the self bias control operates to reduce the initial bias as the signal bias increases. This is for the reason that the increased negative bias from the diode output impedance serves to reduce the anode current and the drop through the self bias resistor 6 I, thereby tending to maintain a more constant bias.

The arrangement is such that the amplifier is provided with signal potentials and two sources of biasing potential, one from the signal which increases with signal strength or carrier wave and another which is applied in the absence of signals and decreases with signal strength or carrier wave.

Referring now to Fig. 2, in which like reference numerals apply to corresponding circuits and elements as in Fig. 1, intermediate frequency signals are received through an input coupling transformer I5 and are rectified by a diode rectifier device 5 in the output circuit of which is provided an output resistor or impedance 23.

Between the output amplifier device 49 and the diode rectifier 5 is provided a high mu type of electric discharge device I05 which is coupled to an output tube 49 through impedance coupling means comprising an anode coupling resistor I'I, a coupling condenser I9, and a grid leak 83. With the high mu type of tube, the anode coupling resistor I0'I is connected through a lead I09 with a high potential tap III on a potential supply resistor 85, while the cathode I I3 is connected with a more negative tap point I I5.

The control grid 1, of the first stage amplifier device I05, is connected through a signal input circuit II 9 including a series resistor I2I, with the movable tap 33 on the diode output impedance 23. The cathode I of the diode rectifier device is connected with a more negative tap I25 on the supply resistor 85 than the tap II for the cathode of the first stage amplifier device I05, whereby a fixed negative initial biasing potential is placed upon the grid H1 in addition to the signal controlled bias potential derived from the diode output resistor 23. The amount of the fixed bias is determined by adjustment of the movable tap I25 with respect to the fixed tap II5.

The circuit arrangement shown in Fig. 2 is, therefore, similar to that shown in Fig. 1 except that a high mu first stage amplifier device is provided whereby a high resistance may be utilized in the anode resistor ID! with a correspondingly high voltage applied to the circuit I09 from the tap I I I for a high signal amplification. Furthermore, a fixed bias is applied to the grid of the amplifier device I 05 instead of the self-bias employed in the circuit of Fig. 1 in connection with the amplifier device 41.

The anode circuit is provided with a suitable filter comprising a resistor H2 and by-pass condenser I I4 between the anode circuit and the cathode. The resistor H2 is of such value as to provide also an appreciable potential drop between the high voltage source B and the anode coupling resistor A.

With an amplifying system of the type herein described whereby the biasing potential for an electric discharge amplifier is derived in part from a fixed source and in part from a diode signal rectifier, together with direct signal application thereto from the signal rectifier output impedance, it has been found that the signal distortion may be reduced to less than 1% when receiving signals with 100% modulation, as compared to a distortion of between 5 and with a diode rectifier and usual amplifier coupling system having a common cathode and without initial bias on the grid of the amplifier tube.

A suitable bias potential which has been found to operate satisfactorily on the grid of the amplifier device, such as shown in Figs. land 2, is substantially one and one-half volts negative on the amplifier control grid in addition to that provided by the diode rectifier circuit.

The system described is, therefore, readily adapted for use in the output circuit of a high fidelity radio signal receiver and materially prevents distortion while providing a diode detector with direct conductive coupling to the following audio frequency amplifier tube without the usual coupling condenser and undesirable load means.

I claim as my invention:

A diode biased amplifier system comprising in combination, a diode rectifier device having a cathode and an anode, means including a signal input circuit for applying modulated signals to said rectifier device, an output resistor connected in circuit with said rectifier device for deriving therefrom alternating current signal and direct current biasing potentials, means providing a variable output contact for said resistor, an electric discharge amplifier device having a control grid directly conductively connected with said contact and having a cathode conductively connected with said output resistor at the positive end thereof, and means connected between said output resistor and the cathode of said amplifier device providing an initial negative biasing potential on said control grid of a value less than normal for amplifier operation and great enough only to insure that the anode of the diode rectifier draws current in response to signals at a more negative potential than the control grid of the amplifier device, and said last-named means being included in the anode circuit of said amplifier device and being of such resistance value that it provides a second source of biasing potential for said control grid which varies automatically with changes in the variable output contact on said diode output resistor.

GEORGE L. BEERS. 

